All cells have mechanosensitive ion channels MSCs). Their physiological roles in non-specialized sensory tissues, such as the heart, are unknown, but new tools have become available. A peptide isolated from tarantula venom specifically blocks a subset of mechanosensitive ion channels, including channels found in heart, kidney, glia and smooth muscle. This proposal addresses the fundamental biophysics of mechanosensitive channel gating, and the molecular properties of the channel viewed through the structure of peptide blockers. The work has direct clinical relevance to stretch induced cardiac arrhythmias, muscular dystrophy and glial tumors. The peptides have been shown to block atrial fibrillation, inhibit stretch induced Ca+2 uptake in dystrophic muscle and stretch-induced secretion of growth factors from glia. Most MSCs open with increases in membrane tension, but mechanisms of activation and inactivation are not clear. In eukaryotes, tension is distributed across the complex mechanical structure of the membrane cortex. This includes the heterogeneous lipid bilayer with transmembrane proteins, the cytoskeleton and the extracellular matrix. Measuring the mechanics of pure bilayers, and natural and simplified cell membranes will show the stresses are distributed. This will reveal new ways channels can be modulated. The major technique is patch clamping. Patch capacitance is a sensitive measure of membrane mechanics. The capacitance changes of defined lipid vesicles under stress will help calibrate the meaning of capacitance changes in natural membranes. Cytoskeleton depleted patches will help define how tension is shared. Current peptide blockers have relatively low affinity, and unknown sites of interaction with target channels. Using recombinant protein expression to exhaustively mutate the peptides we will attempt to find peptides with 1) significantly higher affinity, 2) different affinities for MSCs in different cell types. This knowledge will help pave the way for channel isolation by affinity chromatography and rational drug design.